Certain oil-soluble or oil-dispersible metal core compounds, i.e. compounds having a metal core bonded to one or more ligands, are known as additives (or additive components) for lubricating oil compositions (or lubricants) for improving the composition's properties and performance. The ligand or ligands confer oil-solubility on the compound. For example, certain oil-soluble molybdenum- and sulfur-containing compounds have been proposed and investigated as lubricant additives. U.S. Pat. Nos. 2,951,040; 3,419,589; 3,840,463; 4,966,719; 4,995,996; and 4,978,464 are representative of patent specifications describing molybdenum- and sulfur-containing compounds.
Molybdenum compounds for use as lubricant additives described in the art are principally dinuclear molybdenum compounds, characterised by the oxidation state Mo(V). See, for example, U.S. Pat. No. 5,627,146. Also, EP-A-0 960 178, based on International Patent Application No. PCT IB97/01656, describes use of trinuclear molybdenum compounds as lubricant additives, i.e. characterised by a different oxidation state (Mo(IV)).
Such dinuclear molybdenum compounds may be exemplified by the formula Mo2OxSyL2, and such trinuclear molybdenum compounds may be exemplified by the formula Mo3SkL4, where x+y=4, k is at least 4, and L represents a monoanionic ligand for conferring oil-solubility or oil-dispersability on the compound, a typical example being a dithiocarbamate, frequently referred to as “dtc”.
The above-exemplified compounds have Mo: ligand (L) molar ratios of 1:1 and 3:4 respectively, i.e. the number of moles of Mo never exceeds the number of moles of ligand L. Since the Mo is an active part of the compound, it would be desirable to increase its proportion, relative to ligand L, in order to reduce the raw material cost of making the compounds; also by increasing Mo proportion the decomposition process to the Mo active part may be improved. The art does not describe any such accomplishment, even though it would be beneficial to do so.
The present invention solves the above problem and provides oil-soluble or -dispersible compounds with polynuclear Mo cores whose Mo content exceeds its solubility or dispersibility conferring ligand content.